It is conventional to refer to batteries for use in hybrid and electric vehicles as traction batteries since these batteries are used for feeding electric drives. FIG. 1 illustrates the basic circuit diagram of a battery system 10 comprising such a traction battery 20. The battery 20 comprises a plurality of battery cells 21. For the purpose of simplifying the illustration in FIG. 1, only two battery cells have been provided with the reference symbol 21.
The battery 20 is formed from two battery cell series circuits 22, 23, which each comprise a plurality of battery cells 21 connected in series. These battery cell series circuits 22, 23 are each connected to a battery terminal 24, 25 and to a connection of a service plug 30.
The positive battery terminal 24 is connectable to the battery 20 via a disconnecting and charging device 40, which comprises a switch disconnector 41 which is connected in parallel with a series circuit comprising a charging switch 42 and a charging resistor 43. The negative battery terminal 25 is connectable to the battery 20 via a disconnecting device 50, which comprises a further switch disconnector 51.
In addition, FIG. 2 shows a diagram 60 which illustrates, in very schematized form, the various fault mechanisms 61 of lithium-ion batteries and the consequences 62 thereof. These illustrated fault mechanisms 61 can result in thermal runaway 64 of the battery cells caused by an impermissible increase in temperature 63. In the event of the occurrence of a thermal runaway 64, as a result of an emission of gas 65 which can occur, for example, on opening of a rupture valve as a consequence of increased battery cell internal pressure, a fire in the battery cells 66 or, in an extreme case, even rupture of the battery cells 67 can occur. Therefore, the occurrence of thermal runaway 64 when using the battery cells in traction batteries needs to be ruled out with the greatest possible probability close to 1.
Thermal runaway 64 can occur in the case of overcharging of a battery cell 70, as a consequence of deep discharge of a battery cell 80 during the subsequent charging operation or in the event of the presence of impermissibly high charging and discharge currents of the battery cell which can result from an external short circuit 90, for example. In addition, thermal runaway 64 can also occur in the event of the presence of a battery cell-internal short circuit 100, which can arise, for example, as a consequence of a severe mechanical force effect during an accident 101 or as a consequence of the formation of battery cell-internal dendrites 102, which can arise, for example, in the event of the presence of excessively high charging currents at low temperatures. Furthermore, thermal runaway 64 can also occur as a result of battery cell-internal short circuits which can be caused by impurities in the battery cells resulting during manufacture, in particular by metallic foreign particles 103 present in the battery cells. Thermal runaway 64 can also occur in the event of the presence of impermissible heating of the battery cells 110 which can arise, for example, as a consequence of a vehicle fire or in the event of the presence of an overload of the battery cells 120.
FIG. 3 illustrates the basic circuit diagram of a battery system 10 known from the prior art which comprises a traction battery 20 comprising a plurality of battery cells 21 and a battery management system (BMS) 11. The electronics of the battery management system 11 have a decentralized architecture, in which the monitoring and actuation units 130 formed from the monitoring electronics (CSC electronics) of the battery cells 21 are in the form of satellites, which are each provided for monitoring the function state of one or more battery cells 21 and communicate with a central battery control device (BCU) 140 via an internal bus system 141.
The electronics of the battery management system 11, in particular the monitoring electronics of the battery cells 21, are necessary in order to protect the battery cells 21 from the critical states illustrated in FIG. 2, which can result in thermal runaway. A high degree of complexity is involved in the electronics of the battery management system 11 in order firstly to protect the battery cells 21 from overload due to external causes such as, for example, due to a short circuit in the inverter of an electric drive, and secondly to avoid a situation whereby the battery cells are endangered by malfunction of the electronics of the battery management system 11, such as, for example, by faulty detection of the battery cell voltages by the monitoring and actuation units 130.
As is the case for the battery system 10 illustrated in FIG. 1, in the battery system 10 illustrated in FIG. 3 the traction battery is connectable to a positive battery terminal 24 via a disconnecting and charging device 40 20 and is connectable to a negative battery terminal 25 via a disconnecting device 50. In this case, in each case the same reference symbols have been used for denoting identical or similar components for the battery systems illustrated in FIGS. 1 and 3.
In addition, the central battery control device 140 is designed to actuate the switch disconnector (relay) 41 and the charging switch (relay) 42 of the disconnecting and charging device 40. The actuation of the switch disconnector 41 and the charging switch 42 by means of the battery control device 140 is symbolized by the arrow 142 in the drawing. The central battery control device 140 is also designed to actuate the further switch disconnector (relay) 51 of the disconnecting device 50. The actuation of the switch disconnector 51 by means of the battery control device 140 is symbolized by the arrow 143.
The central battery control device 140 is connected to a respective other battery terminal 24, 25 in each case via a high-voltage line 144, 145. In addition, the central battery control device 140 comprises current sensors 150, 160, which are provided for measuring a current flowing through the traction battery 20. The battery control device 140 also communicates with a vehicle interface via a CAN bus 146. Information relating to the function state of the vehicle can be provided to the battery control device 140 via the CAN bus.
When using a battery management system 11 of a battery system known from the prior art, it is therefore desired to increase the safety of the battery system 10 such that no unreasonable risk occurs. In doing so, pursuant to ISO 26262, stringent requirements are placed on the functional safety of the battery management system 10 since a malfunction of the electronics, as explained already above, can result in a risk. In addition, safety tests are stipulated for lithium-ion battery cells. In order to be able to transport the battery cells, for example, UN transport tests need to be performed. The test results need to be evaluated in accordance with the EUCAR hazard levels. The battery cells in the process need to adhere to prescribed minimum safety levels. In order to achieve this, extensive additional measures are taken in the battery cells which are intended for use in traction batteries.
For battery management systems 11 for battery systems 11 comprising traction batteries 20 for electric vehicles and plug-in hybrids, presumably grading in accordance with the hazard level ASIL C will be established if the safety of the battery cells 21 cannot be significantly increased. Such additional measures will be taken by virtue of the fact that so-called safety devices are integrated in the battery cells. Typically, the safety devices specified below are thus integrated in the battery cells.
An overcharge safety device (OSD) is integrated in a battery cell. Such an overcharge safety device has the effect that the battery cell does not exceed an EUCAR hazard level 4 during an overcharging operation. The permissible range for the battery cell voltage ends at 4.2 V. In the case of an overcharging operation, above a battery cell voltage of approximately 5 V, such a high internal pressure builds up in the battery cell that a membrane of the overcharge safety device curves outwards and the battery cell is electrically short-circuited. As a result of this, the battery cell is discharged until a battery cell-internal fuse is activated. The short circuit in the battery cell between the two poles of the battery cell is maintained via the overcharge safety device.
In addition, a battery cell fuse is integrated in the battery cell. This fuse integrated in the battery cell is a very effective protective instrument on a battery cell level, but causes considerable problems when using the battery cells to construct a series circuit in a battery module or in a battery system. In these cases these measures are rather counterproductive.
A nail penetration safety device (NDS) is also integrated in a battery cell. A nail penetration safety device protects the battery cell by virtue of such a defined short-circuit path which does not result in such severe local heating of the battery cell in the region of the nail penetration that local melting of the separator provided could result being constructed when a nail or a pointed object penetrates into the battery cell.
A safety function layer (SFL) is also integrated in a battery cell. The safety function layer is realized by the ceramic coating of one of the two electrodes of the battery cell, preferably by the ceramic coating of the anode. In the event of melting of the separator, an areal short circuit of the battery cell and therefore extremely rapid conversion of the electrical energy from the battery cell into lost heat can be prevented by means of the safety function layer.
A crush safety device is in addition also integrated in a battery cell. The crush safety device has a similar mode of operation to the nail penetration safety device. In the event of a severe mechanical deformation of the battery cell housing, a defined short-circuit path is provided in the battery cell which prevents severe local heating of the battery cell and thus increases the safety of the battery cell.
In the battery cells under development at present, in particular the measures for the electrical safety which protect against overcharging, for example, or ensure overcurrent protection are associated with considerable complexity. In addition, these measures tend to be even rather counterproductive instead of expedient once a battery cell is used in a battery module or in a battery system. For example, on activation of the fuse of a battery cell, the situation may arise whereby the electronics of the existing battery management system (BMS) are subject to very high negative voltages. This results in additional complexity on the battery system level since the transport regulations at the battery cell level need to be adhered to without another benefit being associated with this.